Well this came out of nowhere. The official twitter accounts for both X-Play and Attack of the Show tweeted a short teaser video where at the end the G4 logo pops up and says 2021 along with the tag line "We never stopped playing".
For those who don't know G4 was a channel that was originally dedicated to all thing videogames. The two most widely known shows were probably X-Play, a game review show with hosts Adam Sessler and Morgan Webb and Attack of the Show, a daily hour long show hosted by Kevin Pereira and Olivia Munn dedicated to the latest news in videogames and pop culture. Unfortunately near the end of the run the channel lost it's way and focused less on game and more on endless re-runs of Cops, Ninja Warrior and Cheaters. The channel finally ceased production and shut down in 2014.
I grew up on this channel and saw it from the old Tech TV days all the way through when I was crying my eyes out having to watch all these amazing people lose their jobs on the final episodes of X-Play and Attack of the Show due to poor corporate management.
No other details have been announced at this time but my guess would be that in this day and age I don't think the channel will be returning as in it's a channel on Direct TV or Cable. A lot has changed in 6 years so I would say this new G4 might be a website or some kind of Youtube account or a Twitch channel with weekly shows and gaming news.